Synchronous machines in the form of turbogenerators are used in power plants to generate electrical energy. For this purpose, a turbogenerator is mechanically coupled to at least one gas turbine and/or steam turbine, the turbogenerator and the gas turbine or steam turbine together constituting a turbo set. Alternatively, a synchronous machine can be operated in a phase-shifter operating mode.
In the case of a turbogenerator, a magnetic direct-current excitation field, required for generating a generator voltage, is generated in the rotor of the turbogenerator.
A static excitation system may be used for the purpose of supplying the excitation windings of a rotor of a turbogenerator. In the case of a static excitation system, an excitation power is transmitted to the excitation windings of the rotor of the turbogenerator via a stationary current converter, stationary carbon brushes and sliprings that are disposed on a rotor shaft of the turbogenerator. This static excitation can react very rapidly to load change, this having an associated speed advantage with respect to regulating technology in comparison with brushless excitation systems. This speed advantage with respect to regulating technology is becoming very important at the present time since, in the course of a global energy transition, the feed-in units are changing, from large power plants for feeding electrical energy into power grids, toward smaller, decentralized regenerative energy generators, and consequently generate a greater dynamic in the power grids. Owing to the fact that the carbon brushes have to be replaced regularly, however, a static energy system is considered to be maintenance-intensive. Furthermore, the supply voltage of a static excitation system is drawn primarily from the generator terminals, such that, during a power grid fault, in the least favorable case, no ceiling voltage can be supplied to the excitation windings of the turbogenerator.
Alternatively, a brushless excitation system may be used to supply the excitation windings of a turbogenerator. In the case of a brushless excitation system, the excitation windings of the rotor of the turbogenerator are connected to the alternating-current windings of a rotor of a main excitation machine, realized as an external-pole machine, via a diode rectifier that rotates concomitantly with the rotor. The external-pole windings of the main excitation machine are fed from a permanently excited auxiliary excitation machine, via a voltage regulator. It is possible to dispense entirely with the maintenance-intensive use of brushes, a brushless excitation system being considered to be virtually maintenance-free. Moreover, unlike the static excitation, a brushless excitation is able to continue to supply the full power to the excitation windings of the rotor of a turbogenerator in the case of a power grid fault. Owing to the large excitation time constant of a brushless excitation system, correction of an excitation current supplying the excitation windings of the rotor of a turbogenerator is effected much more slowly, in comparison with the static excitation, in the case of rapid load changes in the connected power grid. Turbogenerators having brushless excitation are becoming less and less able to fulfill the constantly increasing minimum requirements of the power grid operators for all feed-in units in respect of a dynamic response in the case of load changes, the voltage stability in the supply grid and in the case of transient disturbances. The excitation power generated by an excitation system is usually approximately 0.5% to approximately 5% of the rated apparent power of a turbogenerator.
WO 2013/079761 A1 discloses a rotating electrical machine and a method for magnetizing a rotor of a brushless rotating machine, the method comprising forming a static magnetic field, rotating the rotor of an excitation machine in the static magnetic field for the purpose of generating an alternating current, rectifying the alternating current by means of a controllable bridge disposed on the rotor, receiving control information wirelessly in the rotor, controlling a current intensity by means of the controllable bridge on the basis of the control information, and feeding the current into a magnetizing winding of the rotating electrical machine.
DE 23 66 003 A1 discloses an arrangement for a rotating semiconductor excitation in the case of turbogenerators, in which there is provided at least one current converter wheel, the carrier disk of which is realized as a hub that is connected to the shaft in a rotationally fixed manner and that has an axially extending hollow-cylinder extension. Semiconductor modules, having heat sinks and protective circuitry, are disposed in the inside diameter of the extension. Furthermore, controlled thyristors are provided as semiconductor valves, and the carrier disk of the current converter wheel has a further hollow-cylinder extension, which is opposite the said hollow-cylinder extension, and on the internal diameter of which the control units for the thyristors are disposed in an isolated manner. Attached to the outer casing of the carrier disk is the rotating part of a solid-state, non-contacting signal transmission device, the stationary part of which is fastened in the machine housing.
DE 10 2010 060 998 A1 discloses a brushless synchronous generator having a stator that has at least one main winding, and at least one auxiliary winding for generating an excitation field, and having a rotor that has a main excitation winding with salient-pole geometry. The synchronous generator is distinguished in that the rotor still has an auxiliary excitation winding with non-salient-pole geometry that is attached, with the main excitation winding, to a common armature and is connected thereto via a rectifier bridge disposed on the rotor. In a generator arrangement having such a brushless synchronous generator, at least one capacitor is provided, which constitutes a resonant circuit with the auxiliary winding of the stator.
EP 2 262 101 A1 discloses an arrangement having an electrical generator, and having a steam turbine and an excitation device, the excitation device being realized in such a manner that, during nominal operation, the auxiliary excitation machine is realized as a permanently excited synchronous machine and, in turning operation, the auxiliary excitation machine is realized as a synchronous motor, or turning motor.
EP 0 254 129 A1 discloses a brushless synchronous generator, which is coupled to a constantly excited excitation machine whose voltage is rectified by means of a rectifier and supplied to the excitation winding of the synchronous generator. The voltage changes caused by loads can easily be corrected, in that a Hall sensor, the Hall voltage of which serves to control the excitation current, is disposed in the air gap of the synchronous generator.
US 2012/153904 A1 discloses a generator having a field coil, which generates a magnetic field that induces electricity in a coil arrangement. A field-coil excitation system has a generator that has an output coil arrangement for generating a.c. voltage. A rectifier converts the a.c. voltage into d.c. voltage at two nodes. A capacitor between the nodes realizes a resonant circuit, as a result of which voltage and current oscillate with a predefined phase shift. A switch and the field coil are connected in series between the nodes. A control unit switches the switch to conducting for a predefined time period. Following the end of the time period, the occurrence of a minimal current causes the switch to become non-conducting. The predefined phase shift enables the minimal current to be detected.